Advances in polymerization and catalysis have resulted in the capability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts the choice of polymerization-type (solution, slurry, high pressure or gas phase) for producing a particular polymer has been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive and economically enhanced processes. Especially illustrative of these advances is the development of technology utilizing bulky ligand metallocene-type catalyst systems.
More recently, developments have lead to the discovery of anionic, multidentate heteroatom ligands as discussed by the following articles: (1) Kempe et al., "Aminopyridinato Ligands--New Directions and Limitations", 80.sup.th Canadian Society for Chemistry Meeting, Windsor, Ontario, Canada, Jun. 1-4, 1997; (2) Kempe et al., Inorg. Chem. 1996 vol 35 6742; (3) Jordan et al. of polyolefm catalysts based on hydroxyquinolines (Bei, X.; Swenson, D. C.; Jordan, R. F., Organometallics 1997, 16, 3282); (4) Horton, et. al., "Cati Diamide Ligand: New Alkene Polymerization Catalysts", Organometallics, 1996, 15, 2672-2674 relates to tridentate zirconium complexes; (5) Baumann, et al., "Synthesis of Titanium and Zirconium Complexes that Contain the Tridentate Diamido Ligand [((t-Bu-.sub.6)N--O--C.sub.6 H.sub.4).sub.2 O].sup.2- {[NON}.sup.2-) and the Living Polymerization of 1-Hexene by Activated [NON]ZrMe2", Journal of the American Chemical Society, Vol. 119, pp. 3830-3831; (6) Cloke et al., "Zirconium Complexes incorporating the New Tridentate Diamide Ligand [(Me.sub.3 Si)N{CH.sub.2 CH.sub.2 N(SiMe.sub.3)}).sub.2 ].sup.2- (L); the Crystal Structure of [Zr(BH.sub.4).sub.2 L] and [ZrCl{CH(SiMe.sub.3).sub.2 }L]", J. Chem. Soc. Dalton Trans, pp. 25-30, 1995; (7) Clark et al., "Titanium (IV) complexes incorporating the aminodiamide ligand [(SiMe.sub.3)N{CH.sub.2 CH.sub.2 N (SiMe.sub.3)}.sub.2 ].sup.2- (L); the X-ray crystal structure of [TiMe.sub.2 (L)] and [TiCl{CH(SiMe.sub.3).sub.2 }(L)]", Journal of Organometallic Chemistry, Vol 50, pp. 333-340, 1995; (8) Scollard et al., "Living Polymerization of alpha-olefins by Chelating Diamide Complexes of Titanium", J. Am. Chem. Soc., Vol 118, No. 41, pp. 10008-10009, 1996; and (9) Guerin et al., "Conformationally Rigid Diamide Complexes: Synthesis and Structure of Titanium (IV) Alkyl Derivatives", Organometallics, Vol 15, No. 24, pp. 5085-5089, 1996.
Furthermore, U.S. Pat. No. 5,576,460 describes a preparation of arylamine ligands and U.S. Pat. No. 5,889,128 discloses a process for the living polymerization of olefins using initiators having a metal atom and a ligand having two group 15 atoms and a group 16 atom or three group 15 atoms. EP 893 454 A1 also describes preferably titanium transition metal amide compounds. In addition, U.S. Pat. No. 5,318,935 discusses amido transition metal compounds and catalyst systems especially for the producing isotactic polypropylene. Polymerization catalysts containing bidentate and tridentate ligands are further discussed in U.S. Pat. No. 5,506,184.
Traditional bulky ligand metallocene-type catalyst systems produce polymers that are in some situations more difficult to process into film, for example using old extrusion equipment. One technique to improve these polymers is to blend them with other polymers with the intent to create a blend having the desired properties that each component individually would have. While the two polymer blends tend to be more processable, it is expensive and adds a cumbersome blending step to the manufacturing/fabrication process.
Higher molecular weight confers desirable polymer mechanical properties and stable bubble formation in the production of films. However, this property also inhibits extrusion processing by increasing backpressure in extruders, promotes melt fracture defects in the inflating bubble and potentially, promotes too high a degree of orientation in the finished film. The anionic, multidentate heteroatom containing catalyst systems tend to produce a very high molecular weight polymer. To remedy this, one may form a secondary, minor component of lower molecular weight polymer to reduce extruder backpressure and inhibit melt fracture. Several industrial processes operate on this principle using multiple reactor technology to produce a processable bimodal molecular weight distribution (MWD) high density polyethylene (HDPE) product. HIZEX.TM., a Mitsui Chemicals HDPE product, is considered the worldwide standard. HIZEX.TM. is produced in a costly two or more reactor process. In a multiple reactor process, each reactor produces a single component of the final product.
Others in the art have tried to produce two polymers together at the same time in the same reactor using two different catalysts. PCT patent application WO 99/03899 discloses using a typical bulky ligand metallocene-type catalyst and a conventional-type Ziegler-Natta catalyst in the same reactor to produce a bimodal polyolefin. Using two different types of catalysts, however, result in a polymer whose characteristics cannot be predicted from those of the polymers that each catalyst would produce if utilized separately. This unpredictability occurs, for example, from competition or other influence between the catalyst or catalyst systems used.
Polyethylenes with a higher density and a higher molecular weight are valued in film applications requiring high stiffness, good toughness and high throughput. Such polymers are also valued in pipe applications requiring stiffness, toughness and long-term durability, and particularly resistance to environmental stress cracking.
Thus, there is a desire for a combination of catalysts capable of producing processable polyethylene polymers in preferably a single reactor having desirable combinations of processing, mechanical and optical properties.